Process for the production of magnesium

ABSTRACT

A PROCESS FOR THE PRODUCTION OF MAGNESIUM BY REDUCING MAGNESIUM OXIDE FROM AN OXIDANT CONTAINING A MAJOR PROPORTION OF MAGNESIA WITH REDUCTANT CONTAINING A METALLIC ALUMINUM-SILICON ALLOY, HAVING A RATIO OF SILICON TO ALUMINUM OF AT LEAST 0.8 TO 1, AT A TEMPERATURE OF AT LEAST 1400*C. AND IN THE PRESENCE OF A MOLTEN SLAG OF DEFINED COMPOSITION AND COMPRISING 5-25 PERCENT MAGNESIUM OXIDE, AND LESS THAN 30 PERCENT CALCIUM OXIDE. THE REDUCTANT AND SLAG ARE PREFERABLY CHOSEN HAVING COMPOSITIONS FALLING WITHIN THE AREAS SHOWN IN THE FIGURES. AS A CONSEQUENCE, THE REDUCTANT AND SLAG ARE COORDINATED SUCH THAT MUCH OF THE ALUMINA AND SILICA OF THE SLAG ARE DERIVED BY THE OXIDATION OF THE REDUCTANT.

y 13, 1971 J. M. AVERY 3,579,326

PROCESS FOR THE PRODUCTION OF MAGNESIUM Filed June 26, 1967' 2 Sheets-Sheet 1 F/gu/e/ IMOLTE/VSLAGCOMPOS/T/O/V IO 20 3O 4O 5O 6 70 0 Percent Alum'mum Oxide INVENTOR.

Y JMUHMAI/ery ATTORNEYS 18, 1971 J. M. AVERY 3,579,326

PROCESS FOR THE PRODUCTION OF MAGNESIUM Filed June 26. 1967 2 Sheets-Sheet 2 F/gure 2 REDUCTA/VT COMPOS/T/O/V IO 20. 3O 4O 5O 6O 7O 8O 9O Percent Iron INVENIOR.

United States Patent Filed June 26, 1967, Ser. No. 648,856 Int. Cl. C22b 45/00 U.S. Cl. 75-67 11 Claims ABSTRACT OF THE DISCLOSURE A process for the production of magnesium by reducing magnesium oxide from an oxidant containing a major proportion of magnesia with reductant containing a metallic aluminum-silicon alloy, having a ratio of silicon to aluminum of at least 0.8 to 1, at a temperature of at least 1400 C. and in the presence of a molten slag of defined composition and comprising 5-25 percent magnesium oxide, and less than 30 percent calcium oxide. The reductant and slag are preferably chosen having compositions falling within the areas shown in the figures. As a consequence, the reductant and slag are coordinated such that much of the alumina and silica of the slag are derived by the oxidation of the reductant.

BACKGROUND OF THE INVENTION This invention is concerned with the production of magnesium metal by the metallothermic reduction of magnesium oxide at elevated temperatures. More particularly, it relates to an improved process of the type, in the prior art, wherein metallic silicon, customarily in the form of a ferrosilicon alloy, and magnesium oxide, customarily in the form of calcined dolomite, are caused to react in an electric furnace-condenser system, customarily maintained under a high vacuum.

In such a process, known as the Magnetherm process, the oxidation of the silicon and the reduction of the magnesium oxide take place in the presence of a molten slag bath at temperatures above about 1300- 1400 C. Under the high vacuum, magnesium vapor at very low partial pressure evolves and is subsequently condensed to molten or solid magnesium metal. Periodically spent ferrosilicon alloy and a large quantity of slag are tapped from the furnace in molten form. Since the furnace-condenser system is maintained under high vacuum in order to promote the desired reaction and to vaporize the magnesium product, the periodic removal of slag and spent alloy from the furnace requires that the vacuum be broken and the operation interrupted. The process is therefore essentially a batch operation.

Such a process has been operated only on a small commercial scale, and it appears to be incapable of competing successfully with the method now used for the production of magnesium on a large commercial scale, namely, the electrolysis of molten magnesium chloride.

Another, Well-known metallothermic process for the production of magnesium, the Pidgeon process, is also a batch operation which has been operated only on a comparatively small commercial scale. In this process magnesium oxide, in the form of calcined dolomite, and metallic silicon, in the form of ferrosilicon, are charged into batteries of small externally heated retorts, in which a solid state reaction occurs at temperatures on the order of 1100-1200" C. Under these conditions metallic magnesium is released from the reaction zone as a vapor at very low partial pressure, and it is therefore necessary to maintain the battery of retorts under very high vacuum.

This process appears to be incapable of competing with the electrolytic method for the production of magnesium ice from magnesium chloride, except under special circumstances or for particular reasons.

BRIEF SUMMARY OF INVENTION The object of the present invention is to provide an improved metallothermic process which is fully competitive with known methods for the production of magnesium.

In general terms, the present invention may be characterized as a continuous or batch process operable at or about atmospheric pressure, in which: an aluminumsilicon alloy is used as the metallic reducing agent; a major portion of the magnesium oxide required is supplied as magnesia, rather than dolomitic lime; a relatively high concentration of magnesium oxide is maintained in the slag at all times; and the proportions of metallic aluminum and silicon and the slag composition are mutually coordinated and maintained in such manner that all, or nearly all, of the alumina and silicon dioxide required for slag formation are derived by the reduction of magnesium oxide to metallic magnesium.

According to the present invention, if an aluminumsilicon alloy, containing silicon and aluminum in a ratio of at least about 0.8:1 is used for the reduction of magnesium oxide, from an oxidant containing a major proportion of magnesia, in the presence of a molten slag of a certain composition at temperatures above about l400 C., magnesium vapor can be released, even at atmospheric pressure, in quantities corresponding to very high utilization of the silicon content of the alloy. Apparently, the presence of aluminum as a reactant, in close physical association with silicon and the molten slag as defined herein, stimulates the reductant synergistically to react, to such an extent that magnesium vapor evolves even at atmospheric pressure, thus obviating the necessity of maintaining a high vacuum on the system.

The use of aluminum-silicon alloys of suitable composition thus provides very important advantages over the use of ferrosilicon. A clear advantage is the possibility of carrying out the reaction at or about atmospheric pres sure. Secondly, a major fraction of the silicon content of the reducing alloy may be utilized to reduce magnesium oxide. Moreover, the aluminum oxide as required in the slag may be derived as a side-product of the reaction, whereby the ratio of slag to magnesium metal produced may be beneficially reduced.

I have further discovered that if the magnesium oxide content of the slag is maintained at a relatively high concentration, above about 5 percent and preferably between about 10 and about 20 percent, the tendency of silicon to react with magnesium oxide to produce magnesium is greatly enhanced. The increased rate of reaction of magnesium oxide with both aluminum and silicon substantially increases the productive capacity of a furnace of given size. The high concentration of magnesium oxide in the slag thus plays a vital role in the process of the present invention.

In order to hold the ratio of slag to magnesium metal produced to the lowest practicable ratio, it is desirable also to maintain a relatively high concentration of aluminum oxide in the slag, namely, greater than about 15 percent and preferably in the range of about 25 to 30 percent.

Furthermore, I have found that the advantages of the present process can best be achieved by avoiding high concentrations of calcium oxide in the slag, While using concentrations of silicon dioxide greater than about 25 percent, preferably in the range from about 30 to 45 percent. The concentration of calcium oxide should be 1 Concentrations and ratios employed herein are by weight unless otherwise specified.

maintained at less than about 30 percent and preferably about 10 to 20 percent.

DRAWINGS DETAILED DESCRIPTION OF INVENTION The process of the present invention is characterized by the utilization of an aluminum-silicon alloy to reduce magnesium oxide in the reaction zone of a reducing furnace, at temperatures above about 1400" C. and in the presence of a molten slag bath of the general composition:

Percent Broad Preferable Component range range An optimum composition of the molten slag appears to be about 20 percent calcium oxide, about 15 percent magnesium oxide, about 30 percent aluminum oxide and about percent silicon dioxide.

Alternatively, the composition of the molten slag may be defined by the curves shown in FIG. 1. Thus, the broad range of composition of the molten slag covers those having about 5-25 percent magnesium oxide, and calcium oxide, silicon dioxide and aluminum oxide in proportions represented by curve 10 of FIG. 1. Similarly, the preferred composition of the molten slag comprises about 10-20 percent magnesium oxide and the remaining components in the proportion shown by curve 12 of FIG. 1. It should be noted that the percentages given in FIG. 1, for simplicity, relate to the molten slag without its magnesium oxide content, that is, 5-25 percent and preferably 1020 percent of the total slag. Thus the total of the three components adds up to 100 percent but represents that part of the slag excluding magnesium oxide.

In preferred embodiments of this invention, the molten slag composition is such that the ratio of aluminum and magnesium oxides to silicon dioxide is less than 1.611; the total aluminum and magnesium oxides is less than 50 percent of the slag; and the ratio of calcium and magnesium oxides to silicon dioxide is less than 1.6:1.

Most metallurgical slags have a high content of CaO-at least percent and usually on the order of 50 percent, sometimes even higher. Such slags are known as basic slags, and they are characterized by a relatively sharp melting point and form a fluid slag of low viscosity with little superheat. Slags of the above range of composition, on the other hand, are classed as acidic, and are characterized by a somewhat vague melting point, and form rather viscous, glassy slags which require considerable superheat to achieve lower viscosity.

The range of the composition given above covers a wide range of melting points. Within this range, many combinations are possible, but they must be carefully selected, because it is necessary to produce a slag in which the mixture of oxides is such that a suitable combination of melting point and viscosity is obtained. To this end, a flux such as fluorspar can be added to the slag if desired.

The oxidant charged to the reaction zone is suitably a mixture of oxides such as magnesium oxide and calcium oxide. A major proportion of the oxidant, however, is magnesia, that is, magnesium oxide ore. A minor proportion of the oxidant may be calcined dolomite, an equimolar combination of magnesium oxide and calcium oxide, derived from CaMg(CO However, it is not necessary to use calcined dolomite in the oxidant, and magnesia alone in some instances is satisfactory. In any event the molar ratio of magnesium oxide to calcium oxide in the oxidant is at least 2: 1.

It will be evident that if a magnesitic dolomite stone having a relatively high ratio of magnesium oxide to calcium oxide content is available, it may be desirable to replace ordinary dolomitic lime, in whole or in part, with lime produced from such a stone, thus decreasing the amount of magnesium oxide required as magnesia. On the other hand, if dolomitic lime is unavailable or not desired, it is evident that lime produced from limestone may be used to provide such calcium oxide as may be necessary for formation of a suitable slag.

In the process of the present invention it is highly desirable to maintain in the reaction zone a temperature of at least about 1400 C. to promote good reaction conditions, but temperatures higher than about 1700 C. are undesirable because they create difficult engineering and operating problems. It is therefore desirable to employ a slag whose melting point is not higher than about 1600 C. in order that enough superheat may be applied to impart sufficient fluidity to the slag without the necessity of excessively high temperature. Thus, a temperature of about 1400-1700 C. in the reaction zone is preferred, although in certain instances higher or lower temperatures are suitable and may be desired.

On the other hand slags of relatively high viscosity can be used in the present process because there is in the furnace no bed of solid material through which the slag must find its way in order to reach the tap hole for removal from the furnace. Thus the problem of slag viscosity is not as great as it is in most metallurgical processes, but it is still a factor requiring attention.

In general, the composition of the slag is determined in the present process by the ratio of aluminum to silicon fed as the reducing agent; the degree of utilization of silicon as reductant, which for reasons of economy should be as high as feasible; and the relative proportions of magnesium oxide fed as magnesia and as dolomitic lime.

As employed herein the term aluminum-silicon alloy includes those reductants which, when added to the molten slag in the reaction zone of a reducing furnace, as herein described, provide metallic aluminum and silicon.

The use of Al-Si-Fe alloys as the reductant of this invention may be desirable, particularly in view of the ready availability of such aluminum-silicon alloys. Aluminum in the form of such alloys can be manufactured by electric furnace smelting procedures, which are well known. As the aluminum content of the alloy increases, a small proportion of iron is desirable or, at times, even necessary in order to prevent excessive volatilization of aluminum and silicon from the furnace. It is generally considered that the aluminum content of these alloys is for practical reasons limited to about percent maximum. In such cases the iron content is generally greater than about 5 percent.

On the other hand, for maximum utilization of the silicon content, it is desirable to have a relatively low iron content in the reductant of this invention. But this desideratum must be balanced against the favorable effect of iron content on the cost of producing the reductant alloy, as mentioned above. An iron content of about 10 percent appears to be satisfactory considering these factors, but it may be higher or lower without departing from the spirit of the invention, for example in the range 0 to 20 percent.

Weighing all of these factorscost of production, concentration of aluminum and silicon, utilization of silicon slag composition and properties, ratio of slag to magnesium produced, composition and properties of the slag, quantity and composition of the spent alloythe desired range of alloy composition for use in the process of the present invention is as follows:

Within the above ranges of alloy composition the ratio of silicon to aluminum is at least 0.8:1, desirably greater than 121 and preferably at least 1.4: 1.

Alternatively, the composition of the aluminum-silicon reductant of the present invention may be represented by the three-component graph of FIG. 2. Thus, broadly the reductant composition falls within the area bounded \by curve 20 on FIG. 2. Preferably, the reductant composition falls within the area bounded by curve 22 of FIG. 2.

The following examples are given to show the technical and economic advantages of the process of the present invention, but without limitation upon the scope of the invention.

EXAMPLES The following operations are conducted in an electric reducing furnace coupled with a condensing chamber. The procedure is to charge the furnace with a slag and to supply heat until a proper viscosity is reached at a temperature above about 1400 C., whereupon the oxidant and reductant are charged in small batches. Periodically, the molten slag and ferrosilicon alloy, if any, are tapped. The addition of oxidant and reductant and the tapping of slag and spent ferrosilicon are conducted in such manner that the composition of the slag is maintained substantially constant. The operation is conducted substantially continuously.

The oxidant and reductant charged to the furnace and the tapped slag has the compositions shown in Table I. The reactions takes place at temperatures also shown in Table I, and magnesium vapor may be evolved and condensed at atmospheric pressure. The slag compositions are also shown in Table I.

For some of the examples, the temperature of the molten slag is higher than desirable, e.g., Examples 2, 5 and 6. In such cases a flux can be added to improve viscosity without requiring excessively high temperatures.

In general, it is desirable to adjust the operating conditions of the process of the invention with respect to compositions and proportions of raw materials fed, slag composition, and temperature of the reaction zone so that substantially all of the aluminum and major portion of the silicon of the reductant react with the magnesium oxide to produce magnesium. Preferably the furnace is maintained at or about atmospheric pressure, although vacuum operation is feasible and may in certain instances be preferred.

Under certain conditions it may be desirable to condense the magnesium under a lower pressure, say down to 0.5 atmosphere, by use of an inert gas such as helium, argon or hydrogen to maintain in the closed furnacecondenser system a total pressure of about one atmosphere.

As is well known, titanium and other metallic oxides are sometimes present in raw materials used for the production of an aluminum-silicon alloy, and the corresponding metal is therefore sometimes present in the alloy produced. The presence of such tramp metals does not interfere with the operation of the process of this invention, and may be tolerated or remedied by suitable metallurgical procedures.

Within the scope of the invention it is possible to devise a wide variety of combinations of alloy compositions and slag compositions while still maintaining the de- TABLE I Example 1 2 3 4. 5 6 7 8 9 10 11 12 13 14 15 16 Reductant:

Magnesia 1 1. 79 1. 41 1. 50 1. 66 1. 68 1. 96 1. 46 1. 62 1. 56 1. 73 1. 81 1. 88 1. 99 1. 95 1. 63 1. 61 S1 Dolomitic lime 1 0. 30 1. 14 0. 92 0. 1. 14 0. 15 1. 00 0. 56 0. 72 0. 25 0 0. 39 0 0 0. 83 0. 75

CaO 10 30 25 15 27 5 28 18 22 9 I 0 12 0 0 23 21 MgO 15 10 10 10 20 20 10 10 10 10 10 20 20 18 15 15 A1203. 30 35 30 35 25 35 30 35 25 30 33 25 30 30 30 30 SiO2 45 25 34 40 28 40 32 37 43 51 57 43 52 32 34 Temperature, C 1, 500 1, 625 1, 525 1, 550 1, 625 1, 650 1, 525 1, 600 1, 400 1, 400 1, 500 1, 525 1, 450 1, 560 1, 575 1, 575 Alloy used 1 0. 68 1. 06 0. 98 1. 11 1. 07 1. 01 0. 84 0.83 0.85 O. 85 0. 84 0.83 0.83 0. 84 0. 84 1. 09 Slag produced 1 1. 72 2. 20 2. 05 1. 76 2. 46 1. 76 2. l0 1. 80 1. 91 1. 59 1. 44 1. 91 1. 59 1. 58 2. 10 2. 05 Fe-Siby-product 0 0.36 0.44 0.44 0.4 1 0.44: 0.19 0.19 0.21 0.21 0.21 0.21 0.21 0.21 0.19 0.44 CaO-l-MgO/SiOz. 1. 60 1.06 0.63 1. 68 0. 63 1. 15 0.76 0. 74 0.37 0.17 0. 74 0. 40 0. 34 1. 19 1. 06 Mg+A1203IS102 1. 00 1. 80 1. 18 1. 12 l. 60 1. 37 1. 25 1. 21 0.81 0. 78 0. 75 1. 05 1. 00 1. 92 1. 09 1. 06

l Expressed as unit weight per unit of magnesium produced. 4 56 percent silicon. 2 72 percent silicon. 5 60 percent silicon.

B 75 percent silicon.

From Table I it can be seen that the utilization of the reductants is high, for the ratios of alloy used to magnesium produced (0.7-1.1) are low. Moreover, the ratios of slag produced to magnesium produced (1.6-2.5) are also low. As compared with the prior art processes, wherein the slag ratio is commonly about 6 to 1, the reduced ratio of this invention has of course a very favorable effect on the economics of the process. Furthermore, the ability to produce magnesium at atmospheric pressure makes the present process operable as a continuous process and hence more economical in certain applications.

described, and to substitute a high magnesium oxide-content lime in whole or in part for dolomitic lime and magnesia, all without departing from the spirit of the invention.

Such variations, from the examples, as described immediately above and elsewhere in this disclosure, as well as other variations obvious to those skilled in the metallurgy arts, may be made without departing from the scope of the invention defined in the following claims.

I claim:

1. A metallothermic process for the production of magnesium, which comprises charging a reductant and an oxidant to the reaction zone of a reducing furnace, maintaining the reaction zone of the furnace at a temperature of at least 1400 C., evolving magnesium vapor from the reaction zone, and condensing and recovering the magnesium as a product; wherein said reductant is a metallic alloy comprising about 25-30 percent by weight aluminum, about 40-65 percent silicon and about -20 percent iron, and having a ratio of silicon to aluminum of at least 0.8: 1, said oxidant comprises at least a major proportion by weight of magnesium oxide and has a molecular ratio of magnesium oxide to calcium oxide of at least 2: l, and said reaction zone contains a molten slag which comprises, on a weight basis exclusive of other components, about 5-25 percent magnesium oxide, about 15-35 percent aluminum oxide, about 25-50 percent silicon dioxide, and less than 30 percent calcium oxide, wherein the ratio of aluminum and magnesium oxides to silicon dioxide in the slag is less than 1.6, the aluminum and magnesium oxides comprise less than 50 percent of the slag, and the ratio of calcium and magnesium oxides to the silicon dioxide of the slag is less than 1.6.

2. The metallothermic process of claim 1, wherein the temperature of the molten furnace charge is about 1400- 1700 C., and the furnace is operated at about atmospheric pressure.

3. A metallothermic process for the production of magnesium, which comprises charging a reductant and an oxidant to the reaction zone of a reducing furnace, maintaining the reaction zone of the furnace at a temperature of at least about 1400 C., evolving magnesium vapor from the reaction zone, and condensing and recovering the magnesium as a product; wherein said reductant is a metallic alloy comprising about 30-40 percent aluminum, about 50-60 percent silicon and about percent iron, and having a ratio of silicon to aluminum of at least 0.8:1, said oxidant comprises at least a major proportion by weight of magnesium oxide and has a molecular ratio of magnesium oxide to calcium oxide of at least 2:1, and said reaction zone contains a molten slag which comprises, on a weight basis exclusive of other components, about 5-25 percent magnesium oxide, about -35 percent aluminum oxide, about -50 percent silicon dioxide, and less than about percent calcium oxide.

4. The metallothermic process of claim 3, wherein said molten slag comprises about 10-20 percent calcium oxide, about 10-20 percent magnesium oxide, about 25-30 percent aluminum oxide, and about 30-45 percent silicon dioxide.

5. The metallothermic process of claim 3, wherein the temperature of the molten furnace charge is about 1400'- 1700 C. and the furnace is operated at about atmospheric pressure.

6. A metallothermic process for the production of magnesium which comprises charging to the reaction zone of a reducing furnace a reductant comprising aluminum, silicon and iron in proportions such that its composition falls within the the area defined by curve 22 of FIG. 2;

an oxidant comprising at least a major proportion of magnesium oxide and having a molecular ratio of magnesium oxide to calcium oxide of at least 2:1; and

recovering from said reaction zone magnesium vapor.

7. The metallothermic process of claim 6, wherein said reaction zone contains a molten slag comprising about 5-25 percent magnesium oxide and, as the remainder,

calcium oxide, aluminum oxide and silicon dioxide in proportions such that the composition of the slag exclusive of magnesium oxide falls within the area defined by curve 10 of FIG. 1.

8. The metallothermic process of claim 6, wherein said reaction zone contains a molten slag comprising about 10-20 percent magnesium oxide and, as the remainder, calcium oxide, aluminum oxide and silicon dioxide in proportions such that the composition of the slag exclusive of magnesium oxide falls within the area defined by curve 12 of FIG. 1.

9. The metallothermic process of claim 6, wherein the temperature of the molten furnace charge is about 1400- 1700" C., and the furnace is operated at about atmospheric pressure.

10. A metallothermic process for the production of magnesium, which comprises charging a reductant and an oxidant to the reaction zone of a reducing furnace, maintaining the reacting zone of the furnace at a temperature of at least 1400 C., evolving magnesium vapor from the reaction zone, and condensing and recovering the magnesium as a product; wherein said reductant is a metallic alloy comprising about 25-50 percent by weight aluminum, about 40-65 percent silicon and about 0-20 percent iron, and having a ratio of silicon to aluminum of at least 0.811, said oxidant comprises at least a major proportion by weight of magnesium oxide and has a molecular ratio of magnesium oxide to calcium oxide of at least 2: 1, and said reaction zone contains a molten slag comprising about 10-20 percent calcium oxide, about 10-20 percent magnesium oxide, about 25-30 percent aluminum oxide, and about 30-45 percent silicon dioxide.

11. A metallothermic process for the production of magnesium, which comprises charging to the reaction zone of a reducing furnace a reductant comprising aluminum silicon and iron in proportions such that its composition falls within the area defined by curve 20 of FIG. 2;

an oxidant comprising at least a major proportion of magnesium oxide and having a molecular ratio of magnesium oxide to calcium oxide of at least 2:1;

wherein said reaction zone contains a molten slag comprising about 10-20 percent magnesium oxide and, as the remainder, calcium oxide, aluminum oxide and silicon dioxide in proportions such that the composition of the slag exclusive of magnesium oxide falls within the area defined by curve 12 of FIG. 1; and

recovering from said reaction zone magnesium vapor.

References Cited UNITED STATES PATENTS 2,099,151 11/1937 Vogt -10 2,286,209 6/1942 Kirk 75-67 2,351,488 6/1944 Cooper 75-67 2,396,658 3/ 1946 Hybinnette et al. 75-67 2,847,295 8/1958 Bretschneider et al. 75-67X 2,971,833 2/1961 Artru et al 75-67X 3,129,094 4/1964 Munekata et al. 75-67 3,254,988 6/1966 Schmidt et al. 75-68 3,441,402 4/1969 Magee et al. 75-67 HENRY W. TARRING II, Primary Examiner US. Cl. X.R. 75-10R, 10A

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3579326 Dated y 1 1971 I Julian M. Avery It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 1, Column 7, Line 5 25-30 should read 25-50.

Signed and. sealed this L .th day of January 1972.

SEAL) F Attest:

{ JLDNAHD I LIi' LETGI IIEH, JR. ROBERT GOTTSCL'IALJK Attesting Officer Acting Commissioner 01" l'm uenm FORM pO'1O50O'69) USCOMM-DC eons-ps9 LI 5 GOVERNMENT PRINTING OFFICE I969 flifi' Sl 

